This invention relates generally to printed circuits or encapsulated electronics devices, such a silicon chips, coated with curable resin compositions comprising epoxy resins, cyanate esters, bismaleimides, and a co-curing agent.
The fabrication of electronic printed circuits often requires the fabrication of very fine electrical interconnections, that are as small as 50 microns in diameter, through a dielectric resin. This technology is generally known as microvia technology. Also, in the fabrication of flip chips, ball grid arrays (BGAs) and chip-scale packages, it is necessary to create electrical interconnections in the form of tiny balls or joints with solders or other electrically conductive substances. Generally the electrical interconnections are encapsulated in a non-conductive permanent resin. The properties of the resin chosen for microvia and encapsulated interconnections are critical to the reliability to the electrical device. A number of resins have been used for such applications, such as epoxy resins, acrylates, cyanate esters and bismaleimide-triazine-epoxy resins.
Epoxy resins, which represent some of the most widely used resins, are characterized by easy processability, good adhesion to various substrates, high chemical and corrosion resistance, and excellent mechanical properties. However, epoxy resins have relatively poor performance at high temperatures, have high dielectric constants, and exhibit significant water absorption. Epoxy resins are generally cured by amines and anhydrides. The cured materials typically contain relatively large proportions of hydrophilic groups such as hydroxyl groups which increase water absorption. Epoxy resins thus are sensitive to hydrolysis at high temperature and high humidity. Moreover, the chemical resistance of epoxy resin is not as good as that of cyanate esters and bismaleimides.
Cyanate ester resins have improved performance relative to conventionally cured epoxy resins. Polyfunctional cyanate esters are normally needed to achieve high crosslinking densities and high glass transition temperatures (Tg). Unfortunately, polyfunctional cyanate esters are typically solid or semi-solid at ambient temperatures and thus the formulated resin systems have relatively high viscosities. These resin systems often require significant amounts of solvents.
Another leading thermosetting resin is bismaleimide which is characterized by excellent physical property retention at high temperatures and high humidities and stable (non-fluctuating) electrical properties over a wide temperature range. These properties make bismaleimide particularly suitable for advanced composites and electronics. Bismaleimides are capable of good performance at temperatures of up to about 230xc2x0 C. to 250xc2x0 C. with good hot-wet performance. However, bismaleimide homopolymers are brittle and as a result are susceptible to microcracking. Moreover, the chemical resistance of bismaleimides is poor in the presence of base compounds. Generally, bismaleimide is combined with cyanate ester to create a resin class generally known as BT resins. These resins provide improved glass transition temperature performance and other improved properties as compared to epoxy resins. They are also less expensive than cyanate ester resins. However, the mixture of cyanate esters and bismaleimides exhibits little co-polymerization, therefore, the combination has inferior properties compared to pure cyanate ester or bismaleimide resins.
The art is in need of thermosetting resins demonstrating both high temperature performance and improved physical toughness, especially for microvia and encapsulated electrical interconnect electronics applications, such as printed circuits, flip chips, BGAs and chip scale packages.
This invention relates to a resin system comprising a mixture of epoxy resins, bismaleimides, cyanate esters and low viscosity co-curing agents that can be applied to a printed circuit, a silicon chip or wafer, or other electronic component, encapsulating it with a dielectric. Openings can be created in the encapsulating resin by conventional methods such as laser drilling, photoimaging, plasma, or other techniques known in the art. These openings can be metallized to form highly reliable electrical interconnections. The inventive resin system demonstrates the excellent processability, adhesion, chemical and corrosion resistances, and mechanical qualities normally associated with epoxy resins; the system also exhibits superior physical and chemical properties as well as the stable electrical properties associated with bismaleimides and cyanate esters. All of these are highly desirable characteristics for encapsulants, microvia and interconnection applications.
In one aspect the invention is directed to a curable composition that includes:
(a) a cyanate ester;
(b) a bismaleimide;
(c) a co-curing agent having the structure R1xe2x80x94Arxe2x80x94R2 wherein Ar is at least one aryl moiety, R1 is at least one unsaturated
(d) an epoxy resin;
(e) optionally, a free-radical initiator; and
(f) optionally, a cyanate ester trimerization catalyst.
Preferred curing agents are 2-allylphenyl glycidyl ether and 2,2xe2x80x2-bis (3-ally-4-glycidoxy phenyl) isopropylidene, hereinafter referred to as 2,2xe2x80x2-diallylbisphenol A diglycidyl ether.
The co-curing agent reacts with the cyanate ester, epoxy resin and bismaleimide. The viscosity of the co-curing agent is low enough at room temperature so that no solvent is generally necessary. The crosslinking density of the cured composition can be varied over a wide range by adjusting the relative proportions of each component in the resin mixture.
The invention is based in part on the integration of (i) a glycidyl group, which is reactive with cyanate ester, and (ii) an unsaturated aliphatic group such as an allyl group, which is reactive to bismaleimide, into a co-curing agent molecule. The presence of this co-curing agent in the inventive resin system not only makes it possible to co-cure cyanate ester and bismaleimide, in addition, it reduces the viscosity of the resin system because of the low viscosity of the co-curing agent. Furthermore, the combination of epoxy resin with the cyanate ester by means of well-established curing reactions produces a cured composition with the before mentioned desirable properties. For example, the thermal stability, high temperature performance and hot-wet resistance of the cured inventive resin system are superior to those of conventional amine and anhydride cured epoxy resins. In addition, the uncured resin exhibits excellent processability while the cured resin system demonstrates toughness and chemical resistance that are superior to those from bismaleimide or cyanate ester homopolymers.